Mercury (Hg) having the atomic number 80 is a heavy, silvery element and the only metal that is liquid at ambient temperature and pressure conditions (freezing point −38.8° C.; boiling point 356.7° C.). In addition mercury also has a high vapour pressure.
Mercury occurs in deposits throughout the world and is often associated with copper, zinc, pyrite and lead ores as well as coal. Upon thermally treating these ores or coals containing traces of mercury the mercury is volatized as mercury vapor and carried out with the off-gases.
Mercury and most of its compounds, however, are extremely toxic and must be handled with care. Accordingly, most countries have extremely stringent requirements with regard to the emission of mercury from industrial processes and the presence of mercury in products produced from process gases such as sulphuric acid from SO2-containing gases released in pyrometallurgical processes. Gases containing elementary mercury have constituted one of the great sources of the emission of industrial mercury to the environment and many new gas cleaning processes have been proposed during the last 30 years for eliminating the elementary mercury from such gases. Examples of such removing processes are the ‘Bolkem process’, the use of a selenium filter or a selenium scrubber.
However, the majority of these proposed gas cleaning processes and particular those that are most efficient are technically very complicated and require the use of expensive special apparatus or sophisticated reactants and additives in order to achieve a satisfactory result. One of the few processes that is widely used in practice and that also belongs to the most effective processes and that, therefore, has dominated the marked at least with regard to its application in the metallurgical field is the so called ‘Boliden-Norzink process’ also referred to as the ‘chloride process’ or Calomel Process. This method is very effective in removing mercury from SO2-gas processed to sulphuric acid. A product acid containing less than 0.5 ppm mercury can be produced from a gas containing up to 150 ppm mercury.
The process, of which various embodiments are described in more detail in U.S. Pat. No. 3,849,537, U.S. Pat. No. 4,233,274 and U.S. Pat. No. 4,640,751, is carried out in a plant shown in FIG. 1:
The process gas containing mercury is introduced via line 11 into a packing 10a of a scrubbing tower 10. In the scrubbing tower a solution containing dissolved mercuric chloride (HgCl2) is circulated (pump 14, lines 13, 15, 17). The mercuric chloride reacts with mercury contained in the process gas to form mercurous chloride (calomel) as follows:HgCl2+Hg→Hg2Cl2 
The scrubbing process removes mercuric chloride from the scrubbing solution. If the concentration of mercuric chloride is not maintained, the scrubbing solution will become ineffective. The concentration of HgCl2 in the solution must be measured and if the concentration falls below a given value strong solution must be added. Sampling, measuring and adjustment of concentration must be done by experienced operation personnel. But due to the mix of continuous operation (scrubber) and batch-wise operation (addition of strong solution) the efficiency of the scrubbing process is slightly fluctuating.
To overcome the concentration losses so-called strong solution is added into the scrubbing tower from storage tank 38 via line 39. Strong solution contains HgCl2 in concentrations 10-20 times more than the concentration in the reaction circuit. Mercurous chloride (calomel) formed during the reaction is insoluble and precipitates from the solution.
The overflow from settler 20 is recirculated to the scrubbing tower, via pipeline 50. The process consumes mercuric chloride (HgCl2) continuously. The concentration of the reaction solution is maintained by batch-wise addition of strong solution.
Strong solution is produced in a separate circuit by regeneration of calomel removed from the reaction circuit via the settlers. A so-called chlorination tank 30 is used. Chlorine from bottles 40 is injected via pipeline 41 and injecting device 34 into a circulating stream of pump 32 lines 31, 33, 35 and 36. Chlorine is absorbed in the solution and reacts with mercurous chloride (calomel) to form strong mercuric chloride solution.Hg2Cl2+Cl2→2 HgCl2 
When the required concentration for the strong solution is achieved, it is pumped through line 37 into a storage tank 38 from where it is fed to the scrubbing tower 10.
It takes about two days to generate sufficient strong solution to operate the scrubbing tower 10 for about two weeks. While HgCl2 is produced in the regeneration tank 30, the concentration of the mercury (II) chloride (HgCl2) is measured. If the chlorine concentration drops below a certain level, additional chlorine will be injected. This procedure is repeated until a washing solution with the defined concentration is achieved. As the process kinetics are quite complex, the addition of new chlorine is not done automatically, but by hand.
The production of strong solution is operated as a discontinuous process, while the removal of the mercury from the off-gases is a continuous process. The storage tank 38 serves to couple these two operating modes as it provides a buffer for the continuous addition of washing solution and compensates fluctuations in the mercury content in the off-gases.